Basaltic magma

Partial melting in the mantle leads to the production of basaltic magma, which forms most of the oceanic crust. Examination of the mineralogy of the oceanic crust, which is dominated by olivine, pyroxene, and feldspar, reveals that little water is involved in the production of the oceanic crust. These minerals are all anhydrous, that is, without water in their structure. Dry partial melting of the upper mantle must lead to the formation of oceanic crust. By collecting samples of the mantle that have been erupted through volcanoes, we know that it has a composition of garnet peridotite (olivine + garnet + orthopyroxene). Analyzing samples of this in the laboratory, by raising its temperature and pressure so that it is equal to 62 miles (100 km) depth, shows that 10 percent to 15 percent partial melt of this garnet peridotite yields a basaltic magma.

Magma that forms at 50 miles (80 km) depth is less dense than the surrounding solid rock, so it rises, sometimes quite rapidly (at rates of half a mile, or one kilometer, per day measured by earthquakes under Hawaii). In fact it may rise so fast that it does not cool appreciably, erupting at the surface at more than 1,832°F (1,000°C), generating basaltic magma.

Continue reading here: Andesitic Magma

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